Multi-group transmissions are used as multi-speed transmissions mainly in utility vehicles. They consist of a power split transmission as a transfer case or input transmission, a main transmission as the basic transmission and a range transmission as a rear-mounted transmission. These transmissions are increasingly more frequently configured as automated standard transmissions, such as the Applicant's known ASTRONIC transmission system typically make available a number of 8, 10, 12 or 16 gears. They offer more operating comfort in comparison with conventional manual transmissions, in particular an easier and almost force-free operation, high security against shifting errors, as well as an extended clutch service life. In addition, these automated standard transmission offer smoother gearshifting, as well as a higher degree of transmission efficiency with comparatively lower production costs in comparison with automatic transmissions.
A fundamental disadvantage of this automated standard transmission in comparison to other automatic transmissions is, however, that it does not shift under load, but is subject to an interruption of tractive force during gear changes when shifting the usual shift elements (jaw clutches, synchronizing devices). This can slow the driving speed of the vehicle, depending on the driving situation, in particular with regard to heavy trucks, in such a way that a shift-in takes place immediately after a shifting operation has ended. A standstill can even occur in an extreme case, when driving uphill with heavy load or starting on an uphill.
Known from DE 101 43 994 A1 of the Applicant is a multi-group transmission with transfer case, basic transmission, and range transmission in which the gear transition in the transfer case is greater than the gear transition in the basic transmission, where the front-mounted group functions as a main transmission and the basic transmission group functions as a power split transmission.
The multi-group transmission is configured as a change speed transmission with countershaft design. It has an input shaft that is associated with the front-mounted group having two gears and a main shaft, which is associated with the downstream two-speed, three-speed or four-speed basic transmission, where the gear wheels, which are mounted so as to rotate, for the different gears on a countershaft engage with correspondingly fixed gear wheels on the input shaft or the main shaft. The gear wheels of the input shaft and the main shaft can, in turn, be connected to their shafts (input shaft/main shaft) via shifting mechanisms in a rotationally fixed manner.
The input shaft and the main shaft of this multi-group transmission are arranged co-axially with respect to each other, where a gear wheel that can be connected to the input shaft, via a shifting mechanism in the front-mounted transmission, is arranged on the main shaft so that an operative connection can be produced between the front-mounted transmission and the basic transmission. The range transmission is connected downstream of the axially opposite end of the main shaft. This transmission group is configured as a planetary gear, which is connected to an output shaft of the multi-group transmission and offers two further transmission ratios so that a maximum possible number of gears is obtained from the multiplication of the individual number of gears of the three transmission groups. The highest gear can be configured thereby as a direct gear, that is, with a gear transmission ratio of 1 or as overdrive with a gear transmission ratio of less than 1.
A realization of different transmissions with different numbers of gears and torques is made possible with a relatively low cost and construction expenditure by way of this modular transmission system. The transmission system can be configured as a manual transmission or a partially automated transmission.
It is thereby disadvantageous that the known multi-group transmission has the tractive force interruption known from the standard transmissions, which can lead to the above-described problems during gear changes and high comfort loss during driving operation, especially in utility vehicles.
Also known from DE 101 35 327 A1 is an automated change speed transmission in single-group design, typically with 6 gears, as well as a process for gear change with a transmission such as this, in which a startup clutch is engaged by way of a drive engine only up to a slip limit during a shifting operation. The input shaft is accelerated via the drive engine until it reaches a synchronous speed for the target gear during the shift-in operation. A further speed increase leads to slipping of the clutch so that the speeds of the drive engine and the input shaft are adapted, when the clutch is operated exactly at the slip limit. For this reason, the speed of the drive engine is adjusted, via a controller, in the direction of the synchronous speed of the target gear at the same time as the clutch is disengaged during a shifting operation and the clutch is subsequently engaged up to the slip limit in order to appropriately accelerate the input shaft.
The operation of the clutch at the slip limit makes an improved operating comfort possible, since torque peaks, which are produced by the roadway on the drive train, are not supported on the drive engine, but lead to a slipping of the clutch. In addition, shortened shifting times result, because the clutch is operated at the slip limit, since the synchronous speed of the respective target gear is reached faster and the tractive force interruptions induced by the transmission are shorter for this reason.
An intermediate gear can also be shifted between an original gear and a target gear in a shifting operation during which one or several gears are skipped, where the slip limit of the clutch and the corresponding synchronous speed are then adapted to the intermediate gear.
It is considered disadvantageous that the operation of the startup clutch at the slip limit is subjected to relatively intensive wear due to the frequent gearshifting. The known method is also envisioned only for increasing the speed of one input shaft during shift-in operations, that is, during a desired gear change to a lower gear. A reduction in the duration of the tractive force interruption can be achieved in this way during shift-in, but not during acceleration of the vehicle and upshift operations connected therewith. In addition, the known process for gear change in single group transmissions is not suitable by implication for multi-group transmissions, which have one input shaft and at least one further main shaft.
Based on this background, it is the objective of the invention to create a multi-group transmission with a high operating comfort, which makes a gear change that is virtually free of tractive force interruptions possible, in particular during the change from a lower to a higher gear.